If they're small, you can ship with carriers like UPS and Fed-X. Just pack them well and be sure to insure them for an appropriate amount. I'd say one out of a thousand packages are damaged or lost.
If larger than about a cubic foot and/or heavier than 50 pounds, palletize each speaker and have a crate built around the pallet. Then send 'em via LTL carrier. Still want to be insured, of course, and the risk is still about the same. But palletized packages sent by LTL truck offer different methods to load and unload. This has some advantages on larger packages, and I think makes 'em less prone to having problems on either end.
Ironically, it's often cheaper to ship LTL too, especially for heavy stuff. Then again, the cost of crating usually more than makes up the transport savings in cost.
The buyer is leaning towards having the carrier packing the double box just in case we have to file a claim. It's very likely that the cost of shipping, insurance and boxing will exceed a reasonable cost.
Is there a reason you don't want to use USPS? I know the postal service isn't fancy, but they have reasonable rates that include insurance and a free box. That's who Amazon uses, and they're a billion-dollar company, so there must be something to it. I order a lot off eBay, and those sellers also usually ship with USPS. It can't hurt to price it out.
eBay sellers often use drop-shipping as their entire business model. They are simply ordering from another source, adding a small mark-up, and allowing that source to ship it off. Maybe that's why you're seeing so many USPS packages.
DHL is the only other shipping company I know. I've used them before at work. They did a decent job. I think they were cheaper than FedEx and UPS, which is why we used them.